Do white holes exist? If so, how are they formed? Does time exist in white
holes, since they are inverse black holes?
Submitted by Chanuka from Srilanka

A white hole is indeed the inverse of a black hole, but we don't expect to
find one in real life.

Within a year of Einstein developing his general theory of relativity - and
saying the equations were too hard to be solved - Karl Schwarzschild found the
first solution describing what we now call a black hole. This was the
simplest version of a black hole: very symmetric, sitting in isolation for all
eternity, no matter falling in, not rotating or changing with time. It became
apparent that it had a surface (which got named an event horizon) which you
could get into but not out of. Light couldn't get out either, which is why
the whole thing eventually got named a black hole.

Later, when people developed more sophisticated mathematical tools to look at
such solutions, they realized that wasn't the whole story. The structure of
space and time in this simple case had to have what is called time reversal
symmetry, which means if you let time run backwards everything should look the
same. Therefore if there is a horizon extending into the future which light
can enter but not leave, there has to be another horizon extending into the
past which light can leave but not enter. That would look like the inverse of
a black hole, so they called it a white hole although it's really the
extension of the black hole into the past. (And it gets even weirder: There
is what looks like another universe inside the horizon, although the meaning
of "inside" becomes distorted.) Time would exist inside a white hole,
although since you couldn't get in you'd have to be born in there to measure
it.

But in real life we don't expect to see white holes, because real black holes
are more complicated than this simple solution to the equations of general
relativity. They didn't exist infinitely far back in the past, but rather
formed some finite time ago by collapsing stars. This ruins the time reversal
symmetry, so if you look at past history you don't get the white hole part of
the solution but rather a black hole forming towards the end of the stellar
collapse.